Strong El Niño played a major role in warming the air above the ice, researchers report

An area of West Antarctica more than twice the size of California partially melted in 2016 when warm winds forced by an especially strong El Niño blew over the continent, an international group of researchers has determined.

In the June 15 issue of the journal Nature Communications, they report that the warm spell persisted for more than two weeks in January 2016. Satellite data revealed a mix of melted snow and ice over most of the Ross Ice Shelf—a thick platform of floating ice that channels about a third of the ice flowing from the West Antarctic Ice Sheet into the ocean. While researchers have been gathering evidence for years that warm ocean water is melting West Antarctic ice shelves from beneath, this is one of the first times they’ve been able to document how warm air could also cause widespread melting from above. As it happens, researchers had installed the necessary instruments to investigate these processes in West Antarctica only a few weeks earlier, as part of a study to better understand how clouds affect the amount of energy that reaches the snow surface and influence its temperature.

“We were extraordinarily fortunate to be able to deploy state-of-the art equipment to West Antarctica just before this large melt event occurred,” said Dan Lubin, principal investigator of the Atmospheric Radiation Measurement (ARM)West Antarctic Radiation Experiment (AWARE). Lubin is a research physicist at the Scripps Institution of Oceanography in La Jolla, California and a coauthor of the Nature Communications study. “These atmospheric measurements will help geophysical scientists develop better physical models for projecting how the Antarctic ice sheet might respond to a changing climate and influence sea level rise,” Lubin added. Julien Nicolas, lead author of the paper, is a research associate at the Byrd Polar and Climate Research Center (BPCRC) at The Ohio State University. He’s part of the OSU team that provides weather and climate analysis for AWARE. When Nicolas got a January 2016 alert from the AWARE expedition that the weather at their campsite atop of the West Antarctic Ice Sheet had turned unseasonably warm, he checked to see in the satellite data what was happening to the rest of West Antarctica.

The presence of water in the snow is often hard to detect from visible satellite imagery, especially if clouds block the view. Instead, Nicolas analyzed satellite measurements of the microwave radiation emitted by the snowpack, since dry and wet snows have very different microwave signatures. What he saw during the melting event was an area of roughly 300,000 square miles, including most of the Ross Ice Shelf, that likely contained a mix of snow and water. “What probably happened is that the surface snowpack was able to contain the meltwater, acting as a buffer and preventing the formation of melt ponds and streams that can be common on some Antarctic ice shelves,” Nicolas said, “but we cannot rule out the presence of standing water in many locations.”

What makes this event particularly interesting to scientists is that it took place during one of the strongest El Niño events on record. “This conjunction of events was no coincidence,” he said. During an El Niño, warm waters from the equatorial Pacific Ocean move east. El Niños also favor weather patterns that steer warm air towards West Antarctica, but strong westerly winds that blow over the ocean to the north of the continent usually keep the warmer air at bay. BPCRC senior research associate Aaron Wilson, also a coauthor on the study, used climate models to show that melt events in West Antarctica are more likely to occur during El Niño conditions, especially when westerly winds are weak. What makes this January 2016 event unique, he explained, is that the warming occurred despite strong westerly winds.

“Without the strong westerlies, it’s likely there would have been much more melting,” Wilson said. Coauthor David Bromwich, professor of geography and leader of the Ohio State team, explained it this way: “In West Antarctica, we have a tug-of-war going on between the influence of El Niños and the westerly winds, and it looks like the El Niños are winning,” he said. “It’s a pattern that is emerging. And because we expect stronger, more frequent El Niños in the future with a warming climate, we can expect more major surface melt events in West Antarctica.” More frequent melting would accelerate the general deterioration of the ice sheet, he concluded.

Satellite measurements match model results apart from in the tropics. There is uncertainty with the tropic data due to how various teams correct for satellite drift. The U.S. Climate Change Science Program conclude the discrepancy is most likely due to data errors.

Who’s desperate to find the missing hot-spot? Sherwood’s new paper claims to have found it, but after years of multi-layered adjustments, and now kriging the gaps, and iteratively homogenizing, the results of the new data partly “solve” one problem while creating others. There’s no documented, physical reason for the homogenizing and there’s no new insight gained. The raw data was used by airlines, the military, and meteorologists for years, yet the suggested new results are quite different to the raw data. It’s as if we can’t even measure air temperature properly. Somehow we’ve made multivariate complex models work but not simple temperature sensors? The main problem with the old results was that they didn’t fit the models. Now, after torturing the data, they still don’t.

Twenty-eight million weather balloons had shown by 1999 that the key assumption in the climate models was wrong. Without feedbacks, the models only produce 1.2°C of warming with a doubling of CO2. With feedbacks the simulations ramp that up to a dangerous 3 – 4 degrees C, and water vapor was the most important feedback. It’s just no fun for the Global Worriers without it.

I’ve looked through the paper and find the statistical black box approach they used to be unconvincing. I’ll leave it to others to examine the details of their statistical adjustments, what what the physical reasons for those adjustments might be.

These analysis results would appear to leave very, very little doubt but that EPA’s claim of a Tropical Hot Spot (THS), caused by rising atmospheric CO2 levels, simply does not exist in the real world. Also critically important, even on an all-other-things-equal basis, this analysis failed to find that the steadily rising Atmospheric CO2 Concentrations have had a statistically significant impact on any of the 13 critically important temperature time series analyzed. Thus, the analysis results invalidate each of the Three Lines of Evidence in its CO2 Endangerment Finding. Once EPA’s THS assumption is invalidated, it is obvious why the climate models they claim can be relied upon, are also invalid. And, these results clearly demonstrate–13 times in fact–that once just the ENSO impacts on temperature data are accounted for, there is no“record setting” warming to be concerned about. In fact, there is no ENSO-Adjusted Warming at all. These natural ENSO impacts involve both changes in solar activity and the 1977 Pacific Shift. Moreover, on an all-other-things-equal basis, there is no statistically valid proof that past increases in Atmospheric CO2 Concentrations have caused the officially reported rising, even claimed record setting temperatures. To validate their claim will require mathematically credible, publically available, simultaneous equation parameter estimation work. The temperature data measurements that were analyzed were taken by many different entities using balloons, satellites, buoys and various land based techniques. Needless to say, if regardless of data source, the results are the same, the analysis findings should be considered highly credible.

Plausible reasons for the inconsistencies between the modeled and observed temperatures in the tropical troposphereWe hereby attempt to detect plausible reasons for the discrepancies between the measured and modeled tropospheric temperature anomalies in the tropics. For this purpose, we calculate the trends of the upper-minus-lower tropospheric temperature anomaly differences (TAD) for both the measured and modeled time series during 1979–2010. The modeled TAD trend is significantly higher than that of the measured ones, confirming that the vertical amplification of warming is exaggerated in models. To investigate the cause of this exaggeration, we compare the intrinsic properties of the measured and modeled TAD by employing detrended fluctuation analysis (DFA). The DFA exponent obtained for the measured values reveals white noise behavior, while the exponent for the modeled ones shows that they exhibit long-range power law correlations. We suggest that the vertical amplification of warming derived from modeled simulations is weighted with a persistent signal, which should be removed in order to achieve better agreement with observations.

The weather report for California 8,200 years ago was exceptionally wet and stormy.

That is the conclusion of a paleoclimate study that analyzed stalagmite records from White Moon Cave in the Santa Cruz Mountains published online Jun. 20 in Nature Scientific Reports.

The Golden State’s 150-year stretch of unusually wet weather appears to have been marked by particularly intense winter storms and coincides with a climate anomaly in Greenland ice cores first detected in 1997. Before this “8.2 ka event” was discovered, scientists thought the world’s climate had been unusually stable during the Holocene, the geological epoch that covers the last 11,700 years of Earth’s history.

Since then researchers have associated the distinctive, 3.3-degree Celsius temperature dip in the Greenland ice cores with a catastrophic event: The drainage of two giant glacial lakes (Lake Ojibway and Lake Agassiz) located in northeastern North America caused by the collapse of massive ice sheet that covered much of the continent during the last ice age. In short order, the two lakes dumped enough melt water into the North Atlantic to disrupt the world’s oceanic and atmospheric circulation patterns and raise the sea level by somewhere between two to 10 feet. The tremendous freshwater flood has been associated with an extended cold snap in Europe, increased drought in Africa, weakened monsoons in Asia and strengthened monsoons in South America.

“This is the first high-resolution evidence of the response of the coastal California climate to the most distinctive event in the Holocene. Although the effects appear to have been less severe than in other parts of the world, it provides us with new information about the nature of this global climate event,” said Jessica Oster, assistant professor of earth and environmental sciences at Vanderbilt University, who directed the study.

Oster is a member of a small community of earth scientists pioneering the use of mineral deposits in caves as proxies for the prehistoric climate. Cave formations, including stalagmites and stalactites, can provide valuable information about the climate for the last 600,000 years. They have a built-in clock: The mineral deposits contain radioactive uranium-234 that decays into thorium-230 at a constant rate so the ratio of the two isotopes is determined by the date the mineral deposit formed. Seasonal variations in water seepage produce layers that can be dated with considerable precision. The ratios of other isotopes in the minerals including oxygen and carbon provide information about the temperature and nature of the vegetation in the region at the time the layers formed. Concentrations of trace elements like magnesium, strontium and phosphorus provide information about how wet the environment was.

“Events like this are particularly difficult to study because they are so brief,” said Oster. “Fast-growing stalagmites are particularly good for this purpose because they have very high temporal resolution.”

With a five-year grant from the National Science Foundation, Oster is analyzing stalagmites from two California caves in order to shed new light on the factors that produced megadroughts in the region during the late Pleistocene and early Holocene. During her studies, she discovered a stalagmite that was growing rapidly just before, during and after the 8.2 ka event. By analyzing the oxygen and carbon isotope ratios and the concentrations of the trace elements phosphorus and magnesium in the mineral layers formed from 6,900 to 8,600 years ago, Oster and her collaborators extracted a considerable amount of information about what was going on in the prehistoric California atmosphere.

According to the paper, “…the new record suggests that the 8.2 ka event was associated with a brief period of wetter conditions, potentially arising from increased storminess, and demonstrates a near synchronous climate response to this event on both sides of the Pacific.”

Climatologists are particularly interested in this prehistoric event because it can provide insight into what would happen if global warming reaches a point where glaciers in Greenland and other parts of the globe melt rapidly enough to dump large amounts of fresh water into the ocean. In 2003, for example, the Office of Net Assessment at the U.S. Department of Defense produced a study of prospective climate change specifically based on this event.

Coauthors of the study are Vanderbilt undergraduate student Jansen Gibson and former graduate student Aaron Covey; Warren Sharp from the Berkeley Geochronology Center; Bruce Rogers from the Western Cave Conservancy; and Hari Mix from Santa Clara University.

A team of top-level atmospheric chemistry boffins from France and Germany say they have identified a new process by which vast amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from the sea – a process which was unknown until now, meaning that existing climate models do not take account of it. The effect of VOCs in the air is to cool the climate down, and thus climate models used today predict more warming than can actually be expected. Indeed, global temperatures have actually been stable for more than fifteen years, a circumstance which was not predicted by climate models and which climate science is still struggling to assmilate. In essence, the new research shows that a key VOC, isoprene, is not only produced by living organisms (for instance plants and trees on land and plankton in the sea) as had previously been assumed. It is also produced in the “microlayer” at the top of the ocean by the action of sunlight on floating chemicals – no life being necessary. And it is produced in this way in very large amounts.

New NASA satellite maps show human fingerprint on global air qualityUsing new, high-resolution global satellite maps of air quality indicators, scientists tracked air pollution trends over the last decade in various regions and 195 cities around the globe.

Impact of aerosol emission controls on future Arctic sea ice coverWe examine the response of Arctic sea ice to projected aerosol and aerosol precursor emission changes under the Representative Concentration Pathway (RCP) scenarios in simulations of the Canadian Earth System Model. The overall decrease in aerosol loading causes a warming, largest over the Arctic, which leads to an annual mean reduction in sea ice extent of approximately 1 million km2 over the 21st century in all RCP scenarios. This accounts for approximately 25% of the simulated reduction in sea ice extent in RCP 4.5, and 40% of the reduction in RCP 2.5. In RCP 4.5, the Arctic ocean is projected to become ice-free during summertime in 2045, but it does not become ice-free until 2057 in simulations with aerosol precursor emissions held fixed at 2000 values. Thus, while reductions in aerosol emissions have significant health and environmental benefits, their substantial contribution to projected Arctic climate change should not be overlooked.

Amplification of Arctic warming by past air pollution reductions in EuropeThe Arctic region is warming considerably faster than the rest of the globe1, with important consequences for the ecosystems2 and human exploration of the region3. However, the reasons behind this Arctic amplification are not entirely clear4. As a result of measures to enhance air quality, anthropogenic emissions of particulate matter and its precursors have drastically decreased in parts of the Northern Hemisphere over the past three decades5. Here we present simulations with an Earth system model with comprehensive aerosol physics and chemistry that show that the sulfate aerosol reductions in Europe since 1980 can potentially explain a significant fraction of Arctic warming over that period. Specifically, the Arctic region receives an additional 0.3 W m−2 of energy, and warms by 0.5 °C on annual average in simulations with declining European sulfur emissions in line with historical observations, compared with a model simulation with fixed European emissions at 1980 levels. Arctic warming is amplified mainly in fall and winter, but the warming is initiated in summer by an increase in incoming solar radiation as well as an enhanced poleward oceanic and atmospheric heat transport. The simulated summertime energy surplus reduces sea-ice cover, which leads to a transfer of heat from the Arctic Ocean to the atmosphere. We conclude that air quality regulations in the Northern Hemisphere, the ocean and atmospheric circulation, and Arctic climate are inherently linked.

Reef-building corals thrive within hot-acidified and deoxygenated watersCoral reefs are deteriorating under climate change as oceans continue to warm and acidify and thermal anomalies grow in frequency and intensity. In vitro experiments are widely used to forecast reef-building coral health into the future, but often fail to account for the complex ecological and biogeochemical interactions that govern reefs. Consequently, observations from coral communities under naturally occurring extremes have become central for improved predictions of future reef form and function. Here, we present a semi-enclosed lagoon system in New Caledonia characterised by diel fluctuations of hot-deoxygenated water coupled with tidally driven persistently low pH, relative to neighbouring reefs. Coral communities within the lagoon system exhibited high richness (number of species = 20) and cover (24–35% across lagoon sites). Calcification rates for key species (Acropora formosa, Acropora pulchra, Coelastrea aspera and Porites lutea) for populations from the lagoon were equivalent to, or reduced by ca. 30–40% compared to those from the reef. Enhanced coral respiration, alongside high particulate organic content of the lagoon sediment, suggests acclimatisation to this trio of temperature, oxygen and pH changes through heterotrophic plasticity. This semi-enclosed lagoon therefore provides a novel system to understand coral acclimatisation to complex climatic scenarios and may serve as a reservoir of coral populations already resistant to extreme environmental conditions.

Climate change refuge for corals discovered (and how we can protect it right now)

Refuge could preserve climate-sensitive corals due to environmental gradients that allow for coral acclimatization

WCS scientists have discovered a refuge for corals where the environment protects otherwise sensitive species to the increasing severity of climate change. The bad news is that the reefs are showing signs of being overfished and weak compliance with local fisheries laws needs to be reversed to maintain the fish that help to keep reefs healthy. The scientists describe their findings in the journal Ecosphere.

A tree-ring based drought reconstruction (AD 1760–2010) for the Loess Plateau and its possible driving mechanismsWe have developed a 272-year ring-width chronology of Chinese pine (Pinus tabulaeformis) growing in the Huanglong Mountains, North China. Climatic response analyses revealed that mean January–July Palmer drought severity index (PDSI) has positive effect on the radial growth of pine trees. Based on the relationships, the mean January–July PDSI was reconstructed for the period from 1760 to 2010. The percentage of variance in the data explained by the reconstruction was 41% during the calibration period of 1950–2010. Spatial correlation analyses between the PDSI reconstruction and gridded PDSI data show that the PDSI reconstruction captures regional drought variations over the environmentally-sensitive area linked to the East Asian summer monsoon. Relatively wet periods are identified for AD 1766–1781, 1795–1804, 1811–1821, 1838–1859, 1884–1889, 1909–1914, 1937–1977 and 2003–2008. Dry conditions prevailed during AD 1760–1765, 1782–1794, 1805–1810, 1822–1837, 1860–1883, 1890–1908, 1915–1936, 1978–2002 and 2009–now. There is a reasonable agreement with dry/wet periods previously estimated from tree-ring data of the Kongtong Mountains of the Loess Plateau. Spatial correlation analyses with sea surface temperature in the Pacific Ocean and tropical Indian Ocean indicated that the Asian summer monsoon circulations play a role in modulating drought variations in the study area whereas the effects of the Pacific Decadal Oscillation are relatively strong.

Tree-ring-based reconstruction of drought variability (1792–2011) in the middle reaches of the Fen River, North ChinaWe developed a tree-ring chronology based on 52 ring-width series from 25 Pinus tabulaeformis trees at Tianlong Mountain (TLM) using the signal-free method. TLM is located in the middle reaches of the Fen River, North China, and is influenced by the East Asian monsoon system. Tree growth was highly correlated (0.789) with the Palmer Drought Severity Index (PDSI) from May to July and indicated a drought-stress growth pattern. Therefore, we developed a robust May-July PDSI reconstruction for 1792–2011 that explained 62.3% of the instrumental variance for 1951–2005. Severe drought years determined by the reconstruction are consistent with conditions reported in historical documents. The TLM PDSI reconstruction was consistent with other tree-ring-based hydroclimate reconstructions in North China; thus, it may accurately represent dry/wet changes that occur over a large area. Cyclical spectral peaks at 2–8 years in the reconstructed PDSI may indicate ENSO activity, as suggested by the positive correlation with the western Pacific sea-surface temperatures (SSTs) and the negative correlation with the eastern Pacific SSTs on the inter-annual scale.

Reconstruction of drought variability in North China and its association with sea surface temperature in the joining area of Asia and Indian–Pacific OceanUsing tree-ring data from the northernmost marginal area of the East Asian summer monsoon (EASM) in North China, May–July mean Palmer drought severity index (PDSI) was reconstructed back to 1767 AD. The reconstruction captured 52.8% of the variance over the calibration period from 1945 to 2005 AD and showed pronounced pluvial periods during 1850–1905, 1803–1811 and 1940–1961 and dry periods during 1814–1844, 1916–1932 and 1984–2012. These anomalous periods have previously been reported in other parts of North China. Spatial correlation analyses and comparisons with other hydroclimatic indices in North China indicated that our new PDSI reconstruction could represent spatial and temporal drought variability in this region well. Our work also suggested that the drying tendency currently observed in the northern part of North China (including the study area) is consistent with the weakening of the EASM. Meanwhile the drying trend was seemingly restrained at present in the southern part of North China. Spatial correlation patterns with global sea surface temperature (SST) indicated that the regional hydroclimatic variability in North China was tightly linked to SST over the joining area of Asia and Indian–Pacific Ocean (AIPO), especially over the tropical western Pacific. When SST from prior November to current July (NJ-SST) in the AIPO area was anomalously high (low), the thermal contrast between Asian land and ocean was weakened (strengthened), and the EASM was correspondingly weakened (strengthened), thereby causing droughts (pluvials) in North China. The results of this study do not only provide useful information for assessing the long-term climate change in North China, but also suggest that abnormal variability in NJ-SST over the AIPO area could be used to forecast hydroclimatic conditions in North China.

Study Sheds Light on How Populations Respond and Adapt to Climate Change

Using a relatively new scientific dating technique, a Baylor University geologist and a team of international researchers were able to document—for the first time—a drastic climate change 4,200 years ago in northern China that affected vegetation and led to mass migration from the area.

Steve Forman, Ph.D., professor of geology in the College of Arts & Sciences, and researchers—using a dating technique called Optically Stimulated Luminescence—uncovered the first evidence of a severe decrease in precipitation on the freshwater lake system in China’s Hunshandake Sandy Lands. The impact of this extreme climate change led to desertification—or drying of the region—and the mass migration of northern China’s Neolithic cultures. Their research findings appear in the January 2015 issue of the Proceedings of the National Academy of Sciences and are available online.

“With our unique scientific capabilities, we are able to assert with confidence that a quick change in climate drastically changed precipitation in this area, although, further study needs to be conducted to understand why this change occurred,” Forman said. Between 2001 and 2014, the researchers investigated sediment sections throughout the Hunshandake and were able to determine that a sudden and irreversible shift in the monsoon system led to the abrupt drying of the Hunshandake resulting in complications for the population. “This disruption of the water flow significantly impacted human activities in the region and limited water availability. The consequences of a rapid climatic shift on the Hunshandake herding and agricultural cultures were likely catastrophic,” Forman said.

He said these climatic changes and drying of the Hunshandake continue to adversely impact the current population today. The Hunshandake remains arid and even with massive rehabilitation efforts will unlikely regrow dense vegetation. “This study has far-reaching implications for understanding how populations respond and adapt to drastic climate change,” Forman said. Forman is the director of the Geoluminescence Dating Research Lab in the department of geology.

Distribution and vegetation reconstruction of the deserts of northern China during the mid-HoloceneDesertification is potentially a serious threat to society, and therefore, it is critical to understand how deserts may respond to future climate change. The mid-Holocene (6 ± 0.5 14C ka) was warmer than present, and the distribution of deserts at this time may have implications for understanding their response to future warming. Here we reconstruct the distribution of deserts in northern China during the mid-Holocene by combining data on vegetation type and the sedimentary facies of aeolian deposits. The results demonstrate that during the mid-Holocene, the deserts retreated northwestward to the location of the modern 300 mm isohyet. Most of the Eastern Desert was stabilized with steppe or forest-steppe vegetation, whereas the Western Desert exhibited no significant change and remained mobile, occupied by desert vegetation. The deserts in northern China were greatly reduced during the mid-Holocene because of the enhancement of the East Asian summer monsoon in a warmer climate than today.

Reconstruction of the geographic extent of drought anomalies in northwestern China over the last 539 years and its teleconnection with the Pacific OceanRecent paleo-climatic/environmental studies have resulted in several high-resolution paleo-precipitation/moisture reconstructions in Northwestern (NW) China over extended periods. Nevertheless, those reconstructions are mostly about the climatic history of individual sites, while fine-grained portrayal and analysis of the geographic extent of drought anomalies across the entire NW China are still missing. We based our study on the dryness/wetness grade series of 19 sites in NW China, which are primarily derived from historical documents, to reconstruct the annual geographic extent of drought anomalies in NW China in AD 1470–2008. Our reconstruction reveals the following periods of drought in NW China: the AD 1470s–1490s, 1620s–1640s, 1700s–1720s, 1770s–1790s, 1860s–1870s, and 1910s–1930s.The most extremely dry years were AD 1928 and 1929. In addition, we found that the influence of El Niño Southern Oscillation (ENSO) on the geographic extent of drought anomalies in NW China was non-stationary at the inter-annual to multi-decadal timescale and that the correlation switched from positive to negative since the late ‘Little Ice Age’. We propose that this non-stationary relationship is attributable to the variance of ENSO and the strength of Asian Summer Monsoon. To conclude, we discuss the implications of the above findings within the context of global warming.

The annual summer monsoon that drops rain onto East Asia, an area with about a billion people, has shifted dramatically in the distant past, at times moving northward by as much as 400 kilometers and doubling rainfall in that northern reach. The monsoon’s changes over the past 10,000 years likely altered the course of early human cultures in China, say the authors of a new study.

Researchers from the Lamont-Doherty Earth Observatory and the Chinese Academy of Sciences in Xi’an studied ancient water levels for Lake Dali, a closed-basin lake in Inner Mongolia in the northeast of China. They found that the lake was six times larger and water levels were 60 meters higher than present during the early and middle Holocene — the period beginning about 11,700 years ago, and encompassing the development of human civilization. “I think it is important to emphasize that these spatial fluctuations in the monsoon drive large changes in northern China,” said Yonaton Goldsmith, a graduate student at Lamont-Doherty Earth Observatory and lead author of the paper. “When the monsoon is strong, it shifts northward and northern China becomes green. When the monsoon is weak, the monsoon stays in the south and northern China dries out. Such large fluctuations must have altered the ecosystems in northern China dramatically.”

The study, appearing this week in the Proceedings of the National Academy of Sciences, also ties the shifting monsoon to changes in Earth’s orbit and other periodic changes in the climate system. The study should help scientists understand how the monsoon is affected by those natural cycles, and how a changing climate today might influence the monsoon in the future. Goldsmith said it’s still unclear how the monsoon will react to global warming. One view is that the monsoon should grow stronger, but the area studied has been drying out over recent decades, he said, “so there is still a lot that needs to be done in that region before we can get definitive answers.”

Dali Lake is located near the northwestern limit of the East Asian monsoon, and so would reflect the changes brought about when the monsoon shifted north. The researchers studied outcrops of sediments left behind when the lake was far larger, and used those and other markers to construct a timeline of lake levels, and the fluctuation of rainfall over millennia. They found that the lake reached peak levels around 123,000 years ago, again around 58,000 years ago, and once more between 11,000 and 5,500 years ago. They tie the periodic increases in rainfall to the range of the monsoon shifting north by as much as 400 kilometers. The lake record is “highly correlated” with measurements taken earlier from cave deposits in both northern and southern China.

Between 5,500 and 5,000 years ago, the monsoon weakened and rainfall over northern China decreased by 50 percent, the researchers found. They speculate that this drying triggered a major cultural transition in the region. As they describe it, two early Neolithic societies, the Hongshan culture in North China and the Yangshao culture in central China, collapsed around 5,000 years ago. In central China, the following period saw the rise of more stratified and socially and politically complex societies, including the Longshan culture. Previously unoccupied areas on the eastern margin of the Tibetan plateau were populated. Meanwhile, northeast China experienced a sharp population decline, represented by the Xiaoheyan culture. “These findings show that climate change can have dramatic effects on human societies and highlight the necessity to understand the effect of global warming on rainfall patterns in China and all over the world,” the authors write.

Intense variations in rainfall may have played a role in the collapse of other civilizations. A study led by Lamont scientist Brendan Buckley, published several years ago, suggested that extended drought coupled with changes in the monsoon could have doomed Cambodia’s ancient Khmer civilization at Angkor nearly 600 years ago. Drought is thought to have played a role in the decline of the Classic Maya civilization, too, though in that case, another Lamont study suggests that the Maya themselves contributed to the drought by clearing forests for cities and crops. The Lake Dali paper’s other authors are Wallace S. Broecker, Pratigya J. Polissar and Peter B. deMenocal of Lamont-Doherty; Hai Xu, Jianghu Lan, Peng Cheng, Weijian Zhou and Zhisheng An of the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences; and Naomi Porat of the Geological Survey of Israel.

This work was supported by a Gary Comer Science and Education Foundation grant to Yonaton Goldsmith and Pratigya J. Polissar; Columbia’s Center for Climate and Life; the National Basic Research Program of China Grant 2013CB955900; the External Cooperation Program of Bureau of International Cooperation, Chinese Academy of Sciences Grant 132B61KYSB20130003; and Lamont-Doherty Earth Observatory Contribution no. 8084.

Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescalesThe magnitude, rate, and extent of past and future East Asian monsoon (EAM) rainfall fluctuations remain unresolved. Here, late Pleistocene–Holocene EAM rainfall intensity is reconstructed using a well-dated northeastern China closed-basin lake area record located at the modern northwestern fringe of the EAM. The EAM intensity and northern extent alternated rapidly between wet and dry periods on time scales of centuries. Lake levels were 60 m higher than present during the early and middle Holocene, requiring a twofold increase in annual rainfall, which, based on modern rainfall distribution, requires a ∼400 km northward expansion/migration of the EAM. The lake record is highly correlated with both northern and southern Chinese cave deposit isotope records, supporting rainfall “intensity based” interpretations of these deposits as opposed to an alternative “water vapor sourcing” interpretation. These results indicate that EAM intensity and the northward extent covary on orbital and millennial timescales. The termination of wet conditions at 5.5 ka BP (∼35 m lake drop) triggered a large cultural collapse of Early Neolithic cultures in north China, and possibly promoted the emergence of complex societies of the Late Neolithic.

Holocene climate change in Arctic Canada and GreenlandThis synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the Holocene in Arctic Canada and Greenland. Our synthesis includes 47 records from a recently published database of highly resolved Holocene paleoclimate time series from the Arctic (Sundqvist et al., 2014). We analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records, mostly from datasets that did not fit the selection criteria for the Arctic Holocene database. Combined, we review evidence from a variety of proxy archives including glaciers (ice cores and glacial geomorphology), lake sediments, peat sequences, and coastal and deep-marine sediments. The temperature-sensitive records indicate more consistent and earlier Holocene warmth in the north and east, and a more diffuse and later Holocene thermal maximum in the south and west. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene.The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C on average (n = 11 sites). The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 ka, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time. The spatial pattern of temperature change through the Holocene was likely driven by the decrease in northern latitude summer insolation through the Holocene, the varied influence of waning ice sheets in the early Holocene, and the variable influx of Atlantic Water into the study region.

A history of snowfall on Greenland, hidden in ancient leaf waxes
A surprising trove of data yields indications of increased Arctic snowfall in times of warming

The history of Greenland’s snowfall is chronicled in an unlikely place: the remains of aquatic plants that died long ago, collecting at the bottom of lakes in horizontal layers that document the passing years. Using this ancient record, scientists are attempting to reconstruct how Arctic precipitation fluctuated over the past several millennia, potentially influencing the size of the Greenland Ice Sheet as the Earth warmed and cooled. An early study in this field finds that snowfall at one key location in western Greenland may have intensified from 6,000 to 4,000 years ago, a period when the planet’s Northern Hemisphere was warmer than it is today. While more research needs to be done to draw conclusions about ancient precipitation patterns across Greenland, the new results are consistent with the hypothesis that global warming could drive increasing Arctic snowfall — a trend that would slow the shrinkage of the Greenland Ice Sheet and, ultimately, affect the pace at which sea levels rise.

“As the Arctic gets warmer, there is a vigorous scientific debate about how stable the Greenland Ice Sheet will be. How quickly will it lose mass?” says lead researcher Elizabeth Thomas, PhD, an assistant professor of geology in the University at Buffalo College of Arts and Sciences who completed much of the study as a postdoctoral fellow at the University of Massachusetts Amherst. “Climate models and observations suggest that as temperatures rise, snowfall over Greenland could increase as sea ice melts and larger areas of the ocean are exposed for evaporation. This would slow the decline of the ice sheet, because snow would add to its mass,” Thomas says. “Our findings are consistent with this hypothesis. We see evidence that the ratio of snow to rain was unusually high from 6,000 to 4,000 years ago, which is what you would expect to see if sea ice loss causes snowfall to increase in the region.” The research was published on May 23 in Geophysical Research Letters, a journal of the American Geophysical Union.

Aquatic plant leaf waxes as a record of snowfall

Thomas’ research looks to understand how precipitation changed in the past, with an eye toward better predicting how modern warming will affect the Earth. “We are using the past to see what might happen in the future,” she says. Aquatic leaf waxes are a relatively new tool for completing this work. They reveal information about the seasonality of precipitation — how amounts of ancient summer rain compared to amounts of ancient winter snow. To understand how aquatic leaf waxes function as a historical record, you need to know a little about aquatic plants. In the Arctic, these organisms survive on lake water, and use hydrogen atoms from this water to produce wax coatings on leaves.

These hydrogen atoms are the key to studying precipitation: In years when the ratio of summer rain to winter snow in a region is high, lake water and aquatic leaf waxes end up containing high levels of a rare form of hydrogen called deuterium, which is heavier than “normal” hydrogen. (This is because summer rain holds more deuterium than winter snowfall.) In contrast, in years when snow is relatively abundant, aquatic plants start producing waxes with less deuterium. This is what Thomas and her colleagues saw when they extracted a long, cylindrical sediment sample from a lake bottom in western Greenland. The mud contains ancient leaf waxes, with the oldest at the base of the column and the youngest at the top.

By dating and analyzing thin slices of the sample, the team determined that aquatic leaf waxes had low levels of deuterium from 6,000 to 4,000 years ago. This is exactly what researchers would expect to see if the warm temperatures of that time had fueled sea ice loss, leading to increased Arctic snowfall and a decline in deuterium in lakes, Thomas said. She acknowledged that it’s possible that a drop in summer rainfall accounted for the changes in deuterium, but says a rise in winter snowfall is the more likely explanation, as scientists have found independent evidence that the region was wetter 6,000 to 4,000 years ago.

The research team included UB Associate Professor of Geology Jason Briner; undergraduate student John J. Ryan-Henry from Brown University, the University of Rhode Island and the Roger Williams University School of Law; and Professor Yongsong Huang from Brown University and the Chinese Academy of Sciences. The study was funded by the National Science Foundation and research grants from the Geological Society of America.

Ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Archipelago: Seasonal cycles and decadal trends
Over the past several decades mountain glaciers and ice caps have been significant contributors to sea level rise. Here we estimate the ice mass changes in the Canadian Archipelago, the Gulf of Alaska, and Greenland since 2003 by analyzing time-varying gravimetry data from the Gravity Recovery and Climate Experiment. Prior to 2013, interannual ice mass variability in the Gulf of Alaska and in regions around Greenland remains within the average estimated over the whole data span. Beginning in summer 2013, ice mass in regions around Greenland departs positively from its long-term trend. Over Greenland this anomaly reached almost 500 Gt through the end of 2014. Overall, long-term ice mass loss from Greenland and the Canadian Archipelago continues to accelerate, while losses around the Gulf of Alaska region continue but remain steady with no significant acceleration.

Greenland’s snowy surface has been getting darker over the past two decades, absorbing more heat from the sun and increasing snow melt, a new study of satellite data shows. That trend is likely to continue, with the surface’s reflectivity, or albedo, decreasing by as much as 10 percent by the end of the century, the study says. The culprits are two feedback loops that are created by the melting itself. One of those processes isn’t visible to the human eye, but it is having a profound effect.